Statistical Investigation and Mapping of Monthly Modified Refractivity Gradient over Pakistan at the 700 Hectopascal Level

This study is an effort to investigate the spatial-temporal variability of the modified refractivity gradient at the 700 hPa pressure level over Pakistan and its neighbouring regions of Afghanistan, India, Iran and the Arabian Sea using the remote sensing data of the AQUA (AIRX3STM) satellite from 2008 to 2012. Trapping conditions only found in December were spread over Khyber Pakhtunkhwa (Gilgit-Baltistan, Pakistan) with an average value of -182.042 M/Km and showing Leptokurtic distributions. The lowest monthly average value super-refractive conditions existed in the autumn season with a strong monthly correlation (>0.91 M/Km). A very high monthly correlation (0.9 M/Km) was found for the super-refractive conditions over the whole time period. The largest spatial and temporal normal conditions appeared in January with the average value for normal conditions being 132.72 M/Km (found over Zabul, Afghanistan) with Leptokurtic distributions. During May normal conditions were the smallest in spatial extent over Pakistan, India and Afghanistan, showing Platykurtic distributions. Sub-refractive conditions mostly prevailed at all times. The probability for extreme sub-refractive conditions was very high in 2008-2012. The highest average sub-refractive conditions appeared in the winter and autumn seasons (spread around Quetta and Kalam, Pakistan). The highest monthly average sub-refractive conditions with a value of 1,265,188 M/Km were found in January and spread around the Sarbaz River Iran. Correlations for the existence of sub-refractive conditions varied from 0.8 M/Km (moderate strong) to 0.4 M/Km during the autumn to winter season. Permanent super-refractive conditions existed over Baluchistan from February to September.

Establishment and Data Processing of High-Precision City Subsidence Monitoring Network by GPS Surveying Instead of Leveling

The feasibility of moni-toring the change of city settlementusing GPS surveying instead of level-ing is studied. A fiducial network anda monitoring network are establishedin Ningbo city. Two periods of GPSobservation are completed. Somemeasures are taken during the obser-vation in order to ensure to obtain thehigh-precise height component. TheSaastamoinen model is adopted in thedata processing of the dry componentpart of the tropospheric delay. Thewet component change of the tropo-spheric delay is estimated by stochasticprocesses model. When Bernese soft-ware is used to process the data, themillimeter level precision of heightmeasuring is achieved.

Impact of tropospheric modelling on GNSS vertical precision:an empirical analysis based on a local active network

The troposphere affects Global Navigation Satellite System(GNSS)signals due to the variability of the refractive index.Tropospheric delay is a function of the satellite elevation angle and the altitude of the GNSS receiver and depends on the atmospheric parameters.If the residual tropospheric delay is not modelled carefully a bias error will occur in the vertical component.In order to analyse the precise altimetric positioning based on a local active network,four scenarios in Southern Spain with different topographical,environmental,and meteorological conditions are presented,considering both favourable and non-favourable conditions.The use of surface meteorological observations allows us to take into account the tropospheric conditions instead of a standard atmosphere,but introduces a residual tropospheric bias which reduces the accuracy of precise GNSS positioning.Thus,with short observation times it is recommended not to estimate troposphere parameters,but to use an a priori model together with the standard atmosphere.The results confirm that it is possible to achieve centimetre-scale vertical accuracy and precision with real time kinematic positioning even with large elevation differences with respect to the nearest reference stations.These numerical results may be taken into consideration for improving the altimetric configuration of the local active network.